EGR systems are employed by internal combustion engines to help reduce various engine emissions. A typical EGR system may include a conduit, or other structure, fluidly connecting some portion of the exhaust path of an engine with some portion of the air intake system of the engine to thereby form an EGR path. Different amounts of exhaust gas recirculation may be desirable under different engine operating conditions. In order to regulate the amount of exhaust gas recirculation, such systems typically employ an EGR valve that is disposed at some point in the EGR path.
Systems have been developed to control EGR flow by regulating the amount of exhaust gases that are recirculated under various operating conditions, e.g., by controlling the position of an EGR valve. Some systems include an actuator for opening and closing the EGR valve, wherein the actuator is controlled by software-implemented control logic. Depending on the operating conditions of the engine, the control logic may position the EGR valve to allow varying amounts of exhaust gases to be recirculated.
EGR control systems may utilize closed-loop control or open-loop control. That is, some systems employ a closed-loop control whereby the control system regulates the EGR valve position based on a feedback signal from a device that detects the amount of flow of EGR gases (e.g., a mass flow meter). Such systems may, for example, compare the amount of actual EGR flow indicated by the feedback signal to a desired EGR flow. Such systems may then position the EGR valve to achieve the desired EGR flow. In contrast, open-loop systems do not base control of the EGR valve on detected EGR flow. Rather, such systems may simply operate based on calibrated EGR flowrates vs. EGR valve position.
In many situations, closed-loop control may provide the most accurate control of EGR flow. However, under certain operating conditions, open-loop control may provide a more accurate control of EGR flow. For example, at low flow rates, it may be difficult to accurately detect the actual EGR flowrate. Therefore, neither closed-loop nor open-loop control is optimal for all situations.
Systems have been developed that employ both open-loop control and closed loop control depending on the operating conditions. For example, European Patent Specification EP 0 820 560 B1, issued to Baert et al. (“the '560 document”), discloses a system configured to utilize both open-loop and closed-loop control. However, the system of the '560 document does not distinguish between the type of control at high flowrates and low flowrates. Thus, the system of the '560 document does not solve the above-mentioned difficulty of accurately measuring low EGR flowrates.
The present disclosure is directed at solving one or more of the problems discussed above.